One of the two people accused of killing 14 at a holiday party in California reportedly posted an online statement pledging allegiance to a leader of the Islamic State militant group.

Tashfeen Malik, 27, and her husband, Syed Rizwan Farook, 28, were killed in a shootout with police hours after Wednesday's massacre at a social services agency in San Bernardino, east of Los Angeles.

The attack was the deadliest mass shooting the United States has experienced in three years.

Malik, a Pakistani native who had been living in Saudi Arabia when she married Farook, posted an online statement of support for a leader of ISIL on an account using a name other than her own, CNN reports, citing US officials.

An image of Syed Farook believed to have been used on an online dating site. (Supplied)

The attack in San Bernardino appeared to be inspired by - but not directed by - the militant group.

The investigation has been focused on the motivation for the attack, with officials including President Barack Obama and San Bernardino Police Chief Jarrod Burguan saying it may have been motivated by extremist ideology.

Twenty-one people were wounded in the attack, the worst gun violence in the nation since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut.

Weapons and ammunition carried by suspects involved in a mass shooting, at the scene of a shooting with an officer, in San Bernardino, California. (AAP)

Farook, a US citizen born in Illinois, was the son of Pakistani immigrants, said Hussam Ayloush, head of the Los Angeles area chapter of the Muslim advocacy group Council on American-Islamic Relations.

Christian Nwadike, who worked with Farook for five years, told CBS his co-worker had been different since he returned from Saudi Arabia.

"I think he married a terrorist," Mr Nwadike said.

Investigators are reviewing the couple's computers and mobile phones to see if they had browsed jihadist websites or had contact with militant groups, according to officials in Washington familiar with the investigation.

Police said the couple had two assault-style rifles, two semi-automatic handguns and 1600 rounds of ammunition in their vehicle, with 12 pipe bombs found in their home.